This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Ricin is one of the most potent biological toxins known, and is classified by the CDC as a category B biothreat. Since post exposure treatment is ineffective unless administered within a narrow window of time, vaccination may be the only way to protect against ricin. We have developed a safe and effective vaccine (RiVax(tm)) based on a recombinant mutant that eliminates the toxicities of the A chain. We have completed Phase I of an efficacy trial with the candidate Rivax vaccine in nonhuman primates. Animals (n=6) were immunized either with three (prime, two boosts) or four vaccinations (prime, three boosts) with 100 micrograms of the vaccine adsorbed to alhydrogel or sham-vaccinated with adjuvant only (n=3). Serum antibodies (alpha ricin IgG) and neutralizing capacity of antibodies were performed by ELISA and ricin cytotoxicity assay, respectively. All animals were then challenged by aerosol to a lethal dose (1 LD^50) of ricin toxin. Results indicated poor survival in both of the immunized groups (1/3, 33%;3-vaccination group;0/3, 0%, 4 vaccination group) and 100% lethality in the sham-immunized controls (0/3). The results of this study paired with the results from phase I of this study (1/6;16% survival in immunized animals) suggest that, although alpha ricin IgG endpoint titers were relatively high in all immunized animals (2.0E+04), the neutralizing capacity to ricin holotoxin was relatively low as shown in the in vitro neutralization assay performed in conjunction with the antibody ELISAs. In addition, antibody production and kinetics showed a peak +14 days post prime immunization, with no definable memory response at either of the immunizing boosts. This result suggests that protection may only be conferred when the quality of the antibodies match the quantity produced in vivo. The second phase of this study is ongoing and will incorporate a variation of RiVax immunizing doses and schedule which may stimulate a memory response in the immunized animals and increase the overall protective capacity of the candidate vaccine.